The present invention relates to fluid sampling apparatus and more specifically to holding devices for sensors which are connected to a process fluid stream.
A typical prior art insertion type sensor is shown in FIG. 1. A sensor such as, for example, conductivity cell 10 is located at the end of a hollow shaft 12, as shown, so that conductivity cell 10 can be placed in the moving process fluid stream 14. In this position, a portion of the process fluid stream 14 passes across the conductivity cell 10 through openings 16 in hollow shaft 12. Wiring for the conductivity cell 10 passes through hollow shaft 12 to proper instrumentation. To prevent process fluid stream 14 from escaping from main process conduit 18 wherein it is located, hollow shaft 12 containing conductivity cell 10 is disposed within an outer conduit 20 having a packing gland 22 to prevent leakage and locking device 23 to hold hollow shaft 12 in place, located at the outer end thereof as shown. To allow removal of conductivity cell 10 for replacement or servicing, the common practice is to include a gate valve 24 in outer conduit 20. To remove the conductivity cell 10 from prior art apparatus as represented by FIG. 1, locking device 23 is loosened and hollow shaft 12 withdrawn to the point where the conductivity cell 10 is clear of gate 26 of gate valve 24 and still within packing gland 22. Gate valve 24 is then closed allowing hollow shaft 12 to be removed the rest of the way without the escape of process fluid stream 14. The re-installation of hollow shaft 12 is the reverse of the above described process. Obviously, this apparatus has certain limitations in addition to its two stage removal and insertion process. Not only is the length of the insertion rod (hollow shaft 12) a physical limitation in itself, but in addition, since it must be straight, this technique cannot be used to insert a sensor around a corner.
Another possible prior art technique is disclosed in FIG. 2. In this instance, a bypass path is created to divert a portion of a process fluid stream from the main process conduit 18 through bypass conduit 28 containing the sensor 11. To allow removal and re-insertion of the sensor 11 in such an arrangement, two valves 30 must be supplied to bracket the sensor 11 as shown.